In communication systems, it is often desirable to obtain the covariance of a received signal vector, for example in order to estimate DOA (Direction of arrival). An antenna lobe may then be directed towards a specific user. Today, an array antenna comprising a number of antenna elements, spaced apart approximately λ/2 in order to avoid grating lobes, and an amplifier coupled to each antenna element, are used. Here, λ denotes the wavelength corresponding to the frequency used, for example the centre frequency of the frequency band used. The received signals form an M×1 vector, where M is the number of elements. DOA is calculated by means of the covariance of the received signal vector.
For a so-called sparse array antenna, the distance between adjacent antenna elements exceeds λ/2. Said distance may be several λ.
It may be desirable to use a sparse array antenna to obtain the covariance of a received signal vector, for example in order to estimate DOA, since mutual coupling between antenna elements is lowered and a high resolution is obtained using few antenna elements. Furthermore, the number of receivers/transmitters is lowered. But there is a problem when using a sparse array antenna, since the covariance matrix becomes ambiguous due to spatial undersampling.
There is thus a need for an array antenna arrangement that is adapted for obtaining the covariance of a received signal vector, for example in order to estimate DOA, without said ambiguity, having a lowered number of receivers/transmitters.
Furthermore, the covariance of the received signal vector may be used for several other purposes than estimating DOA, for example estimating the channel and suppression of interference. For these cases, the calculation results in an ambiguity in the same way as described above. There is thus a general problem that is to be solved; to calculate an unambiguous covariance matrix of a received signal vector, with a lowered number of receivers/transmitters.